We propose an open-label pilot study of the changes in weight, BMI, body composition, and lipids, glucose, insulin and other metabolic parameters occurring in subjects as they switch from treatment with olanzapine, risperidone or quetiapine to aripiprazole. We also will determine resting energy expenditure (REE) and respiratory quotient (RQ) as measured by metabolic cart to determine if either energy expenditure or the propensity to store energy as fat may be involved in any changes to weight that are detected. Food intake, hunger, and physical activity will also be assessed.